A joining technique for laminating and bonding a plurality of workpieces is employed in a variety of fields. In an optical field, for example, such joining (joint or bonding) technique is used to fabricate a multilayer (layered, laminated or laminate) optical member, which includes a plurality of laminated workpieces.
Workpieces to be laminated may be glass substrates and/or crystal (crystalline) substrates (e.g., quartz substrates). A pair of a glass substrate and a glass substrate, a pair of a glass substrate and a crystal substrate, or a pair of a crystal substrate and a crystal substrate may be used to provide a multilayer optical member depending upon an intended use. Each substrate may be planar, or have a concave surface or a convex surface.
One example of the optical member is a wavelength (wave) plate or board. The wavelength plate is an optical element that generates a phase difference in the light passing the wavelength plate. The wavelength plate is made from a plurality of birefringent crystal substrates (e.g., quartz substrates) bonded to each other.
An optical low-pass filter may be attached to a front face of an image pick-up (or imaging) device, such as CCD and CMOS, to prevent moire and false colors. The optical low-pass filter may be made from a quartz substrate and a glass substrate bonded to each other, or two quartz substrates bonded to each other.
When an optical lens is fabricated, a plurality of glass lenses having different refractive indexes are bonded to each other to suppress aberrations and obtain necessary optical features.
The joining technique for joining the workpieces such as glass substrates and crystal substrates may be used in fields other than the optical field. For example, a process of fabricating a micromachine such as a piezoresistance semiconductor pressure sensor, an acceleration sensor and an actuator may include a process of joining silicon substrates, which are crystal substrates, to each other. When various quartz devices are manufactured, quartz substrates may be bonded to each other. In the field of electronic devices, a single crystal wafer bonding process may include a similar joining technique.
When the workpieces such as glass substrates and/or crystal substrates are bonded, the following joining techniques are used.
Conventionally, as shown in Patent Literature 1, for example, a bonding (adhesive) agent is applied on joining surfaces of workpieces, and the workpieces are laminated. An ultraviolet curing adhesive is often used as the bonding agent, as disclosed in Patent Literature 2.
Methods of boding glass substrates to each other include, other than the above-described method, a method of providing a low melting point glass frit between the joining surfaces and bonding the substrates by fusion as disclosed in Patent Literature 3, and an optical contact method as disclosed in Patent Literature 4. The optical contact method prepares substrates having precisely polished glass surfaces, and tightly adheres the glass surfaces to each other.
When a crystal substrate (workpiece) is bonded to a glass substrate (workpiece), these two workpieces are laminated (stacked) and heated while being pressurized as disclosed in Patent Literature 5, for example. Although a bonding mechanism is not entirely clear in Patent Literature 5, it is assumed that the workpiece surfaces which are activated by the heating are combined to each other by the pressurizing, and the workpieces are joined to each other.
In another method, the surfaces of the quartz substrates are mirror polished and/or chemically cleaned (washed) to smooth or flat surfaces which are terminated with OH groups (radicals), and then the two substrates (workpieces) are laminated and heated, as disclosed in Patent Literature 6, for example.
In yet another method, an SiO2 film is formed on the joining surface of the glass substrate, and the glass substrate is directly joined to the crystal substrate by means of interatomic bond, as disclosed in Patent Literature 7.
In still another method of boding the workpieces, fine crystal continuous thin films are provided on the joining surfaces of the respective workpieces, the workpieces are laminated, atomic diffusion is forced to occur on the joining interfaces of the fine crystal continuous thin films and the grain boundaries thereof, and the workpieces are joined to each other, as disclosed in Patent Literature 8, for example.
In recent years, use of this atomic diffusion joining method to join the quartz substrates of a quartz device was reported by KYOCERA KINSEKI Corporation and Takehito Shimatsu of Tohoku University.